[0001] The invention relates to a method of making compacted graphite iron.
[0002] Compacted graphite (CG) irons exhibit a graphite shape intermediate between that
of stringy, interconnected flakes in gray iron and the dispersed, disconnected spheroids
in ductile iron. In many ways, CG irons combine the better properties of both gray
and nodular iron into one material. The yield strength approaches that of ductile
iron while the material retains the machining properties and castability of gray iron.
CG irons have been recognized as early as 1966 (see U.S. patent 3,421,886). However,
the introduction of commercial CG iron has been inordinately slow.
[0003] The chemistry of CG iron is essentially that of nodular iron except that, in processing,
the nodularizing agent, such as magnesium, is either added in smaller proportions
or is allowed to fade prior to casting, or Ti is added, so that the graphite formation
is changed to that of a compacted configuration as opposed to a spheroid. As used
herein, "fade" means a diminution in the effectiveness of the nodularizing agent in
accordance with the progression of time. The chemistry of a typical nodular iron is
3.2-4.1% carbon, 1.7-2.8% silicon, .45-.8% manganese, .1-.14% phosphorus, .05-.13%
sulfur. In a commercial nodular iron, magnesium is used as a treatment element and
is retained in the final casting in an amount of about .04% and sulfur is reduced
to about .002%; in a CG iron, the magnesium may be retained in amount of about .01-.03%.
[0004] Gray cast iron is the least expensive of all the cast metals. This is due to the
type of raw materials used: pig iron, cast iron scrap, steel scrap, limestone, coke
and air, all of which are relatively inexpensive. Gray cast iron is commercially used
primarily in the as-cast condition, whereas nodular iron (which requires specialized
nodularizing treatment) is used in an as-cast, annealed, or normalized condition and,
in some cases, it is quenched and tempered.
[0005] It is helpful to compare some of the existing or known physical properties of commercial
gray iron and commercial nodular iron with known CG irons which have not been significantly
commercialized (see Table 1 below).

[0006] It would be extremely desirable if a compacted
CG iron could be formulated which continued to exhibit the good physical characteristics
of thermal conductivity, shrinkage, and damping similar to that of known
CG irons while at the same time have highly improved strength and hardness characteristics
approaching that of nodular cast iron. In other words, to approach the combination
of characteristics as boxed in Table 1 would be desirable.
[0007] The prior art has attempted to increase or optimize certain of the physical characteristics
of such iron. In an effort to provide a bainitic/austenitic iron, the prior art has
employed the use of certain alloying ingredients, in one case (U.S. patent 3,860,457)
to promote strength characteristic of a bainitic microstructure in nodular iron, and
in a second case (U.
S. patent 3,549,431) to promote an increase in thermal expansion in gray iron, also
characteristic of a bainitic structure.
[0008] In U.S. patent 3,860,457 a nodular iron was produced (magnesium is .03 or greater);
the addition of molybdenum and nickel was made to promote pearlite and thereby, in
conjunction with the bainite, produce a highly increased strength level. Unfortunately,
the use of molybdenum and nickel as pearlite promoters in a nodular iron tends to
sacrifice and decrease thermal conductivity, shrinkage and damping, physical characteristics
which are of keen interest to this invention. These characteristics are detrimentally
injured substantially as a result of the addition of molybdenum and nickel in the
amounts recited. It should also be mentioned that molybdenum is generally accepted
in the art as a pearlite destroyer during heat treatment, contrary to the teaching
of U.S. patent 3,860,457, and thus the teaching of this patent is suspect.
[0009] In U.S. patent 3,549,431, a gray cast iron was produced which had increased thermal
expansion as a result of the addition of elements which included nickel and molybdenum.
However, since the thermal expansion proved to be relatively low compared to that
of CG irons and, therefore, one cannot deduce that the use of nickel and molybdenum
would have any favourable effect upon thermal conductivity, shrinkage or damping now
sought to be maintained along with an increase in strength and hardness. In fact,
the addition of nickel and molybdenum to a gray cast iron tends to reduce the thermal
conductivity, shrinkage and damping characteristics from the levels normally enjoyed
with a conventional gray cast iron.
[0010] This invention is a method by which the strength and hardness of CG iron castings
can be dramatically increased and, at the same time, maintain the present levels of
thermal conductivity, shrinkage and damping characteristics typical of known CG iron.
In particular, the method is an economical way of making high strength CG iron parts
by essentially alloying the iron melt with nickel, molybdenum and magnesium, and at
least one of titanium and/or cerium followed by an austempering heat treatment after
solidification.
[0011] According to the invention there is provided a method of making compacted graphite
iron, comprising; (a) forming a ferrous alloy melt consisting essentially of, by weight,
3-4.0% carbon, 2-3% silicon, .2-.7% manganese, .25-.4% molybdenum, .5-3.0% nickel,
up to .002% sulfur, up to .02% phosphorus, and impurities or contaminants up to 1.0%,
the remainder being essentially iron, said melt being subjected to a graphite modifying
agent in an amount and for a period of time effective to form compacted graphite particules
upon solidification; (b) solidifying said melt to form a CG iron casting; and (c)
heat treating said iron casting by austempering to produce an iron having a matrix
of bainite and austenite.
[0012] Graphite modification may be carried out by use of magnesium in an amount that will
provide .015-.04% in the casting, and titanium and/or cerium in amounts that will
provide in the casting .08-.15%.
[0013] Preferably, the molybdenum is maintained at a level of about .3% and nickel at a
level of about 1.5% to optimize the strength and hardness characteristics. The carbon
equivalent for said iron melt if maintained in the range of 4-4.75; Cu may be added
in an amount of .4-1.9% to maintain the carbon in the matrix of the casting microstructure.
Advantageously, the austempering treatment involves heating to an austenitizing temperture
of 1500-1700°F,holding the melt at said temperature for .5-4 hours, and tempering
by cooling in a low temperature salt bath to a temperature level of 450-800°F, holding
at the latter temperature for .5-4 hours, then cooling to room temperature.
[0014] The composition resulting from the practice of the above method is essentially bainitic/austenitic
compacted graphite cast iron consisting essentially of 3.0-4.0% carbon, 2-3% silicon,
.2-.7% manganese, .01- .02% magnesium, .25-.4% molybdenum, .5-3.0% nickel, sulfur
up to a maximum of .002%, and phosphorus up to a maximum of .02%, 30% austenite, and
70% bainite. The composition has a tensile strength of 100-130 ksi, yield strength
of 85-110 ksi, a shrinkage characteristic significantly less than nodular iron, and
the ability to be cast in a thin wall casting of down to .06 inches thick.
[0015] The invention will now be described by way of example with reference to the accompany
drawings in which:
Figures 1 and 2 are photomicrographs (respectively 100X and 500X) of solidified bainitic/austenitic
compacted graphite irons made in accordance with this invention; and
Figure 3 is a graphical illustration of thermal treatment used to produce the iron
of Figures 1 and 2.
[0016] Developmental
CG irons are commonly produced by the use of commercial graphite modifiers in the form
of magnesium or cerium, the latter being made as additions in very small, regulated
amounts to the melt prior to solidification. When the magnesium or cerium content
in the solidified structure is above about .025%, nodular graphite usually precipitates.
Flake graphite is formed at magnesium concentrations below about .015%. Accordingly,
with magnesium or cerium concentrations in the range of .015-.025%, compacted graphite
(otherwise sometimes referred to as vermiculite) will precipitate. The addition of
titanium to magnesium or cerium treated irons makes it possible to produce compacted
graphite irons in both medium and heavy castings at higher magnesium or cerium concentrations.
The presence of titanium reduces the amount of control required on the magnesium concentration
and is of considerable benefit in compacted graphite formation. Thus, with a magnesium
addition containing titanium, compacted graphite will form with magnesium or cerium
concentrations in the range of .015-.035%, possibly even up as high as .04%.
[0017] The invention herein provides a method by which a
CG iron can be modified to increase the strength and hardness values above that obtained
with conventional processing while at the same time preserving the level of shrinkage,
thermal conductivity, and damping characteristics normally enjoyed with a conventional
compacted graphite iron. To this end, the method of this invention essentially comprises:
(a) casting an iron alloy melt into substantially the shape of the desired part, the
melt consisting essentially of, by weight, 3.0-4.0% carbon, 2.0-3.0% silicon, .2-.7%
manganese, .25-.4
% molybdenum, .5-3.0% nickel, and no greater than .002% sulfur and .02% phosphorus,
with impurities up to 1% and the remainder iron, said melt having been subjected to
graphite modifying agent to form compacted graphite particles upon solidification;
and (b) heat treating the cast part to provide an austempered bainitic/austenitic
compacted graphite microstructure having 30% austenite and 70% bainite, with 12% by
volume compacted graphite being present. The cast part will have a tensile strength
of 100-130 ksi, a yield strength of 85-110 ksi, a fracture elongation of 5-7%, a hardness
of 240-320 BHN, a thermal conductivity of .1, a damping characteristic having a ratio
of .6, and a shrinkage significantly less than nodular iron when cast into a thin
wall of about .06 inches.
[0018] The melting is typically performed in a furnace heated to 2800-2850°F, and then teamed
into a treating ladle at a temperature of about 2750°F. Alloying elements are added
to the treating ladle along with graphite modifiers in the form of magnesium and titanium.
Commercial graphite modifying agents may comprise (a) rare earth elements added to
a desulfurized iron, or (b) Mg and
Ti added prior to post-inoculation (slightly higher base sulfur can be used). Mg is
used in an amount to provide .QL5-.04% in the casting and Ti is used in an amount
to provide .08-.15% in the casting. The treated melt is then poured into one or more
pouring ladles, and at each of the pouring ladles a post-inoculant in the form of
ferro-silicon or ferro-silicon with aliminum and calcium is added. The melt is then
poured into molds at a temperature in the range of 2500-2600°F and the mold cooled
without any special cooling treatment. The graphite modifying agent may be added in
a commercially available form which typically has a composition of 52% silicon, 10%
titanium, about .9% calcium, 5
% magnesium, .25% cerium, the modifier is added in an amount of about .5% of the total
melt. The post-inoculant added to the pouring ladle comprises ferro-silicon or titanium
bearing ferro-silicon added in an amount of about .5%. Thermal treatment of the solidified
or cast melt is shown in Figure 3.
[0019] Copper may be added to the melt in an amount of .4-1.9% to maintain the carbon in
the matrix of the casting microstructure. It is preferred that the melt chemistry
be maintained at optimum percentages, including about 3.6% carbon, about 2.7% silicon,
about .3% manganese, about .02% magnesium, about .1% titanium, about .7% copper, about
.3% molybdenum, and about 1.5% nickel.
[0020] This method provides the ability to obtain higher strength and hardness values for
a compacted graphite iron while at the same time preserving the thermal conductivity,
shrinkage and damping characteristics normally obtained. The importance of this contribution
is made clear by reference to Table I, which presents physical characteristics obtained
for various iron samples to compare conventional compacted graphite iron (sample 1)
which had been subjected to an austenitizing and tempering treatment, and samples
2-6 wherein Ni and Mo had been added in varying amounts to gray iron and given the
indicated austemper treatment. Table I also compares the addition of nickel and molybdenum
to a conventional gray iron melt (sample 7) as well as to a conventional nodular iron
melt (sample 8), and one sample (sample 9) compares the elimination of the austempering
treatment. Improved physical characteristics are not obtained except when a critical
amount of nickel and molybdenum is added to a compacted graphite iron and subjected
to an austempering treatment as previously disclosed. Each of the samples was prepared
with a base chemistry of 3.6% carbon, 2.5% Si, .5% Mn, .01% phosphorus, .001 sulfur.
The melt was heated in accordance with the preferred mode and cast at a pouring temperature
of 2550°F. Each casting was subjected to a heat treatment as indicated in Table I
at temperatures listed.
[0021] It can be seen from Table I that sample 2, representing the CG iron invention herein,
obtained a tensile strength level of 110 ksi, a yield strength of 90 ksi, a hardness
of 285 BHN, along with a thermal conductivity of .1-.12 Cal/cm S°C, a shrinkage value
of .9-1.0, and a damping characteristic of .6. A similar conventional gray iron, sample
1, without the presence of nickel and molybdenum, obtained only a tensile strength
level of 50-80 ksi, a tensile strength of 42-70 ksi, an elongation of only 3%, a hardness
level of 140-270 BHN, and a thermal conductivity retained at .1-.12, and the excellent
shrinkage damping characteristics of conventional CG iron were also retained. Sample
2 had a mixture of pearlite, austenite and bainite. When a conventional nodular iron,
sample 8, contained nickel and molybdenum amounts similar to that used in the invention
herein, the thermal conductivity, shrinkage and damping characteristics suffered in
that they dropped to lower levels.
[0022] When insufficient Mo was added, sample 3, the casting suffered in that only pearlite
was formed accompanied by lower strength and elongation. When insufficient Ni was
added, sample 5, the casting contained pearlite again accompanied by poorer elongation.
When excess Mo or Ni was added, samples 4 and 6 respectively, the casting suffered
in that martensite was formed accompanied by much poorer elongation in 4 and lower
strength levels in 6. Sample 9 illustrates the significant reduction in thermal conductivity,
increased shrinkage, and poorer damping when the austemper treatment is eliminated.

1. A method of making compacted graphite iron, comprising; (a) forming a ferrous alloy
melt consisting essentially of, by weight, 3-4.0% carbon, 2-3% silicon, .2-.7% manganese,
.25-.4% molybdenum, .5-3.0% nickel, up to .002% sulfur, up to .02% phosphorus, and
impurities or contaminants up to 1.0%, the remainder being essentially iron, said
melt being subjected to a graphite modifying agent in an amount and for a period of
time effective to form compacted graphite particules upon solidification; (b) solidifying
said melt to form a CG iron casting; and (c) heat treating said iron casting by austempering
to produce an iron having a matrix of bainite and austenite.
2. A method as claimed in Claim 1, in which said melt is heated to a temperture of
2800-2850op prior to solidification.
3. A method as claimed in Claim 1 or 2, in which said graphite modifying agent to
which said melt is subjected comprises magnesium in an amount that will provide .015-
.035% of said agent in the casting.
4. A method as claimed in Claim 3, in which said graphite modifying agent also includes
titanium in an amount of .1-.15% permitting said magnesium to be present up to .04%.
5. A method as claimed in any way one of the preceding claims in which molybdenum
is present in an amount of about .3% and nickle about .5%
6. A method asclaimed in any one of the preceding claims, in which copper is additionally
added to said melt in the range of .4-1.9%, said copper being effective to maintain
the carbon in the matrix of the casting microstructure.
7. A method as claimed in any one of the preceding claims, in which said melt has
a carbon equivalent in the range of 4-4.75%.
8. A method as claimed in any one of the preceding claims in which said austempering
heat treatment is carried out by heating the casting to an austenitizing temperture
in the range of 1500-1700°F, maintaining said temperture for a period of .5-4 hours,
quenching the casting in a salt bath to a temperature level of 400-8000F for a period of .5-4 hours, and then cooling the casting to room temperature.